Inside, they found what law enforcement sources describe as a massive, sophisticated hydroponic grow house. The feds seized 450 mature marijuana plants — worth $2.5 million — and arrested the two men believed to be running it.

Traditionally, a hydroponic grow house cultivates three to four crops a year, sources said, meaning the suspects were running a $10 million a year operation.

The suspects have been identified as Keith Levine, 33, of Bayard Street in Brooklyn, and Michael Arroyo, 37, of Alpine Avenue in Sunnyside.

Utility records show the lab was in operation for the past three years, sources said, and agents found a ventilation and fertilization system inside.

The investigation started about eight days ago, when a cooperating source tipped the task force to the existence of the grow house, sources said.

Staten Island Advance/Jan Somma-HammelThe garage sits toward the very end of Veterans Road West, between the borough's Animal Care and Control receiving center and a private horse stable.

When the DEA showed up and started doing surveillance work, nearby neighbors approached them to say they had smelled marijuana from the building, sources said.

The garage sits toward the very end of Veterans Road West, between the borough’s Animal Care and Control receiving center and a private horse stable.

The stable’s owner, Kate Agugliaro, said she had never seen any activity by the building. "I thought it was abandoned," she said."

A rental car agent who works across the street also recalled no activity around the building, but he noticed something in the air.

"I park all the cars over there, and I always thought it smelled like pot, but I thought, ‘Nah,’" he said. "On days that it’s windy, when the (rental center) garage doors are always open, it smells like pot."

Still, he added, "I’ve never seen one person anywhere near that thing."

The task force — which includes members from the NYPD and New York State Police — moved in yesterday to make the arrests.

Today, the garage remained shuttered. Only a faint odor of marijuana remained in the air, though a stray marijuana leaf could be seen on the ground, amid the discarded blue evidence gloves.

As of this evening, the U.S. Attorney’s Office were still writing up a formal complaint against the two suspects, though they’re expected to be charged in U.S. District Court in Brooklyn.

The bust marks the second major marijuana operation on Staten Island to fall to authorities over the past month and a half. On March 1, authorities raided what they referred to as a $1 million-a-year hydroponic grow house operation in Rosebank.

NYPD detectives found 316 marijuana plants and 60 seedlings in the 20 James Pl. home of Craig and Keith Harrigan, 49 and 47, police allege. The brothers stole $150,000 in electricity to keep the grow lamps running, police said. Police arrested both brothers, as well as Craig Harrigan’s son, Marc, 18.

DEA Special Agent in Charge John Gilbride said his agency has been finding "more and more" indoor labs in the New York area over the past few years. Using hydroponically-grown pot can mean higher profits for local dealers, who don’t have to pay for drugs that are smuggled in from Mexico or Jamaica, he said.

The labs are typically "very sophisticated," he said, with significant preparation going into electrical use, lighting and soil type.

"They’re trying to produce a plant that has the highest level of THC," Gilbride said, referring to the main psychoactive ingredient in marijuana. And since the lab can run 24 hours a day, "you can have basically a grow operation that produces four crops a year."

Each fully mature plant can yield up to a pound of marijuana, which can sell to wholesalers for $3,500 to $5,000, Gilbride said.

Stopping this type of marijuana-growing operation remains a top priority of the DEA, Gilbride said.

"Marijuana is the most abused drug, and it is a drug that has the potential of generation an enormous amount of profit for criminal organizations," he said. "It’s millions and millions of dollars that are being used... to foster any other type of criminal enterprise."